NASA's Astrobiology Portal has published a series of six Great Debates as prominent scientists sift through the available evidence to reach what are sometimes directly opposing conclusions. The first debate will explore the factors required to make a planet habitable and the question of whether complex life like that on Earth is common or rare in our galaxy.

When the book Rare Earth was published two years ago, it raised a great deal of controversy among astrobiologists. Written by Peter Ward and Donald Brownlee, the book's hypothesis suggests complex life is rare in the universe, and may even be unique to Earth. If life does occur elsewhere, the authors contend, it will only be in the form of single-celled life such as bacteria.

The "Rare Earth" hypothesis is directly opposed to the cultural assumption that there are many alien civilizations, derived from the famous estimate by Frank Drake - known as the "Drake Equation" - that was later amended by Drake and Carl Sagan. They arrived at an estimate that there are perhaps a million intelligent civilizations in the Milky Way Galaxy alone.

The participants in the Great Debate include Donald Brownlee and Peter Ward, co-authors of Rare Earth and professors at the University of Washington, Frank Drake, and David Grinspoon, the 2006 Carl Sagan Award winner and astrobiologist.